The use of truck mixers is known to transport concrete from a production plant to the construction site in which the concrete is used.
Both during the loading step in the production plant and also during the transport step toward the construction site, the concrete must be kept malleable, and therefore the rotating drum which contains the concrete must be kept in constant rotation so as not to cause the concrete to dry out and harden.
In known truck mixers, the rotation of the rotating drum normally occurs by means of a hydraulic engine, which is moved by a group of pumps commanded by a heat engine which is usually a diesel engine. The heat engine can be the same one that moves the vehicle or an auxiliary engine independent from that of the vehicle.
In the patent application IT-MI2012A000093, in the name of the present Applicants, a drum is described for a truck mixer driven by an electric drive member fed by electric accumulators.
The electric accumulators are charged by absorbing energy from the heat engine of the vehicle when it is driven, or by absorbing energy due to braking of the vehicle or again by means of connection to the electricity grid.
One disadvantage of this type of truck mixer is that, if the drive members of the drum suffer a mechanical failure, the drum cannot be kept in rotation if the breakdown is not fixed in a short time, and therefore the mix can solidify inside the drum.
In this eventuality, the drum of the truck mixer is damaged to the point of having to be replaced, which causes problems both from the point of view of maintenance and of replacement costs.
Some solutions to overcome this problem have been proposed in the state of the art.
DE 3214405 A1 shows for example a drive device to empty a mixing drum in emergency conditions, which provides the possibility of disconnecting the main hydraulic engine, if it breaks down, and of connecting a compressed air motor in its place. This document does not make reference to the use of electric motors and in any case provides the need to disconnect the non-functioning engine and to connect an emergency auxiliary motor in its stead.
Other documents which show the use of an electric motor to drive the drum are DE 20 2009 001416U1 and DE 10 2004 038504 A1.
One purpose of the present invention is to obtain a truck mixer of the type discussed above and equipped with a safety device which prevents damage to the drum in conditions of mechanical failure of the devices that make the drum rotate.
Another purpose of the present invention is to obtain a truck mixer equipped with a simple and economic safety device to be installed and activated in conditions of mechanical failure of the main engine of the truck mixer.
The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the art and to obtain these and other purposes and advantages.